Magmatic H2O Research Articles

Geology and Genesis of the Tuwu Porphyry Cu Deposit, Xinjiang, Northwest China

Abstract The Tuwu porphyry Cu deposit is located in the northern segment of the Jueluotage metallogenic belt in Eastern Tianshan, on the southern margin of the Central Asian orogenic belt, Xinjiang, northwest China. Tuwu is hosted by diorite porphyry and tonalite porphyry intrusions, which intruded volcanic rocks of the Carboniferous Qi’eshan Group. Four stages (I-IV) of hydrothermal activity have been identified. Chalcopyrite is the dominant ore mineral and mainly occurs in vein stages II (quartz-chalcopyrite-pyrite ± sericite ± bornite ± enargite veins with phyllic halos) and III (quartz-molybdenite-chalcopyrite ± pyrite ± chlorite ± epidote veins). Re-Os dating of molybdenite samples yielded an isochron age of 335.6 ± 4.1 Ma (2σ, mean square of weighted deviates = 0.15, n = 8). Silicon, oxygen, and carbon stable isotope compositions of quartz and calcite provide evidence for predominantly magmatic contributions with a late meteoric water component at Tuwu. Chalcopyrite samples from stages I and III record a narrow range of bulk δ34S values between −3.9 and 0.4‰, whereas pyrite samples from stages I to IV show decreasing δ34S values from 1.7 to 0.2‰. Chalcopyrite has 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios similar to those of porphyry intrusions, and Pb isotope data from sulfide samples display a positive trend transecting the growth curves of crustal lead. The ore-forming components (metals and sulfur) were sourced from a mantle-derived magmatic reservoir with some upper crustal components in a subduction-related arc setting. Plagioclase compositions in the porphyry intrusions are consistent with magmatic H2O contents of ~7 wt %. Copper sulfides in the high-grade phyllic alteration zones at Tuwu are characterized by elevated δ65Cu values consistent with deposits from oxidized and hydrous magmatic-hydrothermal fluids, whereas lower δ65Cu values and low copper grades correspond to the potassic alteration zones. Recognition of copper isotope zonation patterns at Tuwu has potential applications in the exploration of porphyry Cu deposits.

Mineral equilibrium constraints on the feasibility of diffusive H2O‐fluxed melting in the continental crust

AbstractGeneration of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H2O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low‐P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H2O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H2O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H2O added. remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher . Our results demonstrate that, at the site of melting, melting is driven by diffusion of H2O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water‐rich fluid phase. Melting will initiate and proceed as long as a gradient exists between the H2O source and target lithology. Our calculations show that an ordinary magma, such as an I‐type magma with typical H2O content, has a high enough to be a viable H2O source, allowing diffusive H2O‐fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H2O, which necessitates a continuously advecting H2O source such as a magma conduit or melt‐bearing shear zone. A magmatic H2O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear‐zone hosted magma conduits are localized features, diffusive H2O‐fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.

The fumarolic CO2 output from Pico do Fogo volcano (Cape Verde)

The Pico do Fogo volcano, in the Cape Verde Archipelago off the western coasts of Africa, has been the most active volcano in the Macaronesia region in the Central Atlantic, with at least 27 eruptions during the last 500 years. Between eruptions fumarolic activity has been persisting in its summit crater, but limited information exists for the chemistry and output of these gas emissions. Here, we use the results acquired during a field survey in February 2019 to quantify the quiescent summit fumaroles' volatile output for the first time. By combining measurements of the fumarole compositions (using both a portable Multi-GAS and direct sampling of the hottest fumarole) and of the SO2 flux (using near-vent UV Camera recording), we quantify a daily output of 1060±340 tons CO2, 780±320 tons H2O, 6.2±2.4 tons H2S, 1.4±0.4 tons SO2 and 0.05±0.022 tons H2. We show that the fumarolic CO2 output from Pico do Fogo exceeds (i) the time-averaged CO2 release during 2015-type recurrent eruptions and (ii) is larger than current diffuse soil degassing of CO2 on Fogo Island. When compared to worldwide volcanoes in quiescent hydrothermal-stage, Pico do Fogo is found to rank among the strongest CO2 emitters. Its substantial CO2 discharge implies a continuous deep supply of magmatic gas from the volcano’s plumbing system (verified by the low but measurable SO2 flux), that becomes partially affected by water condensation and sulphur scrubbing in fumarolic conduits prior to gas exit. Variable removal of magmatic H2O and S accounts for both spatial chemical heterogeneities in the fumarolic field and its CO2- enriched mean composition, that we infer at 64.1±9.2 mol. % H2O, 35.6±9.1 mol. % CO 0.26±0.14 mol. % total Sulfur (S ), and 0.04±0.02 mol. % H2.

Pre- and syn-eruptive conditions of a basaltic Plinian eruption at Masaya Volcano, Nicaragua: The Masaya Triple Layer (2.1 ka)

The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style.By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080 and 1100 °C and 21–42 MPa, with a pre-eruptive volatile content of ~2 wt% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8–1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20 and 50 vol%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1–5 min, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt%, storage at moderate temperatures of 1080–1100 °C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.

Experimental constraints on the differentiation of low-alkali magmas beneath the Tonga arc: Implications for the origin of arc tholeiites

Hydrous melting experiments on a basaltic andesite from Late Volcano in the Tonga Arc were conducted at 900–1250 °C and 1 atm to 2.5 GPa, with a range of added H2O concentrations (0–9 wt%). These were used to constrain conditions of phenocryst precipitation in the basaltic andesite and to better understand the processes of magmatic differentiation beneath Tongan volcanoes. Comparison between the products of experiments and Tongan lavas indicates that basaltic andesites from Late crystallised plagioclase + augite + pigeonite ± orthopyroxene while water-saturated at comparatively low pressures (≤0.2 GPa) but high temperatures (≥1000 °C). Glasses produced at 950 °C and 0.2 GPa are similar in composition to dacites from neighbouring volcanoes, including Fonualei. This confirms previous interpretations of dacite magma genesis by crystal fractionation of plagioclase and clinopyroxene from basaltic andesite parent magmas, rather than by partial melting of lower crust. This produces decreases in Dy/Yb with increasing SiO2 without a role for amphibole, probably due to the high temperatures and low alkali concentrations involved. Tongan magmas are primarily tholeiitic, and their high water contents bring into question previous hypotheses that attribute the tholeiitic trend to low concentrations of magmatic H2O. Attributes inherited from the Tongan parent magmas, including low alkali concentrations, appear to drive tholeiitic differentiation, regardless of water content.

Petrogenesis and metallogenic implications of volcanic rocks from the Lawu basin, eastern Tibet: Insights into the intracontinental Eocene-Oligocene porphyry copper systems

Generally, porphyry Cu deposits are associated with the comagmatic porphyry (or subvolcanic)-volcanic systems of high magmatic H2O-fO2 conditions. The volcanic rocks, as the counterpart of the porphyries, thus can provide some significant insights into the fertility of the porphyries to some extent. For this reason, we have used the Lawu volcanic rocks and spatial-temporal closely-related porphyries in the newly discovered porphyry Cu prospects (e.g., Seli, Zongguo, Mamupu) in the southern segment of the Yulong intracontinental porphyry Cu belt to illustrate the relationship between the porphyries and volcanic rocks, petrogenesis of the volcanic rocks, and then to evaluate the ore potential of the porphyries based on the magmatic H2O-fO2 conditions.Both the Lawu volcanic rocks and Seli-Zonguo-Mamupu porphyries are shoshonitic and metaluminous, and have similar REE patterns, and Sr-Nd-Hf isotopic compositions, which suggest a comagmatic relation between the volcanic rocks and porphyries. An episodic magmatism model is proposed to explain the slightly younger age (~36–35 Ma) and less evolved nature of volcanic rocks than the porphyries. The Lawu volcanic rocks of mainly intermediate composition (SiO2 = 54.25–64.68 wt%) have high K2O (4.75–5.94 wt%) and high K2O/Na2O ratios (1.69–2.00), broadly similar to the coeval Yulong fertile granitic porphyries and the Nangqian mafic lavas. The (87Sr/86Sr)i and εNd(t) values, uniform zircon εHf(t) and δ18O values, and lack of inherited zircons of the Lawu volcanic rocks don’t support their formation by mixing between the mantle-derived Nangqian mafic lavas and crust-derived Yulong felsic porphyries or assimilation and fractional crystallization (AFC) of mafic magmas. They are characterized by high Ba/Th, Ba/La and listric-shaped normalized rare earth element profile with significantly negative Nb-Ta-Ti anomalies, and have high initial 87Sr/86Sr ratios (0.7071–0.7079) and low εNd(t) values (−5.71 to −3.05), and low zircon εHf (t) (−1.53 to 4.09) and clearly high δ18O values (6.67–8.42‰), suggesting that, they were probably formed by fractional crystallization (FC) of mantle-derived mafic magmas and originated from mantle domains modified by significant amount of H2O-rich marine sediments of the Paleo-Tethyan oceanic slab.Magmatic H2O contents calculated from deepest-crystallized amphiboles indicate that, the Lawu volcanic rocks and Zongguo porphyries have initial magmatic H2O contents as high as the Yulong fertile porphyries and typical porphyry Cu systems worldwide (commonly >4 wt% H2O). Magmatic fO2 (ΔFMQ) of the Lawu volcanic rocks (0.6–1.3, ave. 0.9 ± 0.1) and the Zongguo porphyries (0.9–1.7, ave. 1.4 ± 0.2) are clearly lower than the fertile porphyries in the giant Yulong deposit (ΔFMQ = 1.6–3.3, ave. 2.3 ± 0.5) and typical porphyry Cu deposits in the world (commonly ΔFMQ > 2). The slightly lower magmatic H2O contents and slightly higher magmatic fO2 of the Zongguo porphyries than the Lawu volcanic rocks were ascribed to variable degassing during magmatic evolution. These suggest that, in spite of the high magmatic H2O contents, the comagmatic porphyries (at least the Zongguo porphyries) of the Lawu volcanic rocks in the southern segment of the Yulong porphyry Cu belt are unlikely to produce large-scale porphyry Cu mineralization like the giant Yulong deposit, due to the low magmatic fO2 conditions.

Origin of giant post‐collisional porphyry Cu metallogenic belt in southern Tibet: constrains from magmatic H2O, fO2, and S

Origin of giant post‐collisional porphyry Cu metallogenic belt in southern Tibet: constrains from magmatic H2O, <i>f</i>O2, and S

Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions

We reconstruct crustal structure along the Lesser Antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions. Xenoliths show considerable island-to-island variation in xenolith petrology from plagioclase-free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole, indicative of changing magma differentiation depths. Xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km. We use xenolith mineral modes and compositions to calculate seismic velocities (vP,vS) and density at the estimated equilibration depths. We create a five-layer model of crustal structure for testing against receiver functions (RF) from island seismic stations along the arc. Lowermost layer (5) comprises peridotite with physical characteristics of mantle xenoliths from Grenada. Uppermost layer (1) consists of 5 km of volcaniclastics and sediments, whose physical properties are determined via a grid inversion routine. The three middle layers (2) to (4) comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island. By inversion we obtain a petrological best-fit for the RF on each island to establish the nature and thicknesses of layers (2) to (4).Along the arc we see variations in the depth and strength of both Moho and mid-crustal discontinuity (MCD) on length-scales of tens of km. Moho depths vary from 25 to 37 km; MCD from 11 and 32 km. The Moho is the dominant discontinuity beneath some islands (St. Kitts, Guadeloupe, Martinique, Grenada), whereas the MCD dominates beneath others (Saba, St. Eustatius). Along-arc variability in MCD depth and strength is consistent with variation in estimated magmatic H2O contents and differentiations depths that, in turn, influence xenolith lithologies. A striking feature is steep, along-arc gradients in vP similar to those observed at other oceanic arcs. These gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle. We find no evidence for large, interconnected bodies of partial melt beneath the Lesser Antilles. Instead, the crustal velocity structure is consistent with magma differentiation in vertically-extensive, crystal mush-dominated reservoirs. Along-arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge, itself driven by variation in slab-derived H2O fluxes.

Probing layered arc crust in the Lesser Antilles using receiver functions.

Oceanic arcs can provide insight into the processes of crustal growth and crustal structure. In this work, changes in crustal thickness and composition along the Lesser Antilles Arc (LAA) are analysed at 10 islands using receiver function (RF) inversions that combine seismological data with vP/vS ratios estimated based on crustal lithology. We collected seismic data from various regional networks to ensure station coverage for every major island in the LAA from Saba in the north to Grenada in the south. RFs show the subsurface response of an incoming signal assuming horizontal layering, where phase conversions highlight discontinuities beneath a station. In most regions of the Earth, the Mohorovičić discontinuity (Moho) is seismically stronger than other crustal discontinuities. However, in the LAA we observe an unusually strong along-arc variation in depth of the strongest discontinuity, which is difficult to explain by variations in crustal thickness. Instead, these results suggest that in layered crust, especially where other discontinuities have a stronger seismic contrast than the Moho, H–k stacking results can be easily misinterpreted. To circumvent this problem, an inversion modelling approach is introduced to investigate the crustal structure in more detail by building a one-dimensional velocity–depth profile for each island. Using this method, it is possible to identify any mid-crustal discontinuity in addition to the Moho. Our results show a mid-crustal discontinuity at about 10–25 km depth along the arc, with slightly deeper values in the north (Montserrat to Saba). In general, the depth of the Moho shows the same pattern with values of around 25 km (Grenada) to 35 km in the north. The results suggest differences in magmatic H2O content and differentiation history of each island.

Melt inclusion constraints on volatile systematics and degassing history of the 2014–2015 Holuhraun eruption, Iceland

The mass of volatiles emitted during volcanic eruptions is often estimated by comparing the volatile contents of undegassed melt inclusions, trapped in crystals at an early stage of magmatic evolution, with that of the degassed matrix glass. Here we present detailed characterisation of magmatic volatiles (H2O, CO2, S, Fl and Cl) of crystal-hosted melt and fluid inclusions from the 2014–2015 Holuhraun eruption of the Barðarbunga volcanic system, Iceland. Based on the ratios of magmatic volatiles to similarly incompatible trace elements, the undegassed primary volatile contents of the Holuhraun parental melt are estimated at 1500–1700 ppm CO2, 0.13–0.16 wt% H2O, 60–80 ppm Cl, 130–240 ppm F and 500–800 ppm S. High-density fluid inclusions indicate onset of crystallisation at pressures ≥ 0.4 GPa (~ 12 km depth) promoting deep degassing of CO2. Prior to the onset of degassing, the melt CO2 content may have reached 3000–4000 ppm, with the total magmatic CO2 budget estimated at 23–55 Mt. SO2 release commenced at 0.12 GPa (~ 3.6 km depth), eventually leading to entrapment of SO2 vapour in low-density fluid inclusions. We calculate the syn-eruptive volatile release as 22.2 Mt of magmatic H2O, 5.9–7.7 Mt CO2, and 11.3 Mt of SO2 over the course of the eruption; F and Cl release were insignificant. Melt inclusion constraints on syn-eruptive volatile release are similar to estimates made during in situ field monitoring, with the exception of H2O, where field measurements may be heavily biased by the incorporation of meteoric water.

The 2011–2012 paroxysmal eruptions at Mt. Etna volcano: Insights on the vertically zoned plumbing system

The activity of Mt. Etna volcano from January 2011 to April 2012 was characterized by 24 paroxysmal, short-duration (from a few to several hours) eruptions at the New South-East summit crater. Despite the violence of the activity, no appreciable geophysical signals were recorded during this period, except for an increase in seismic tremors just minutes/hours before the occurrence of the paroxysm. This type of activity represents a significant shift from the mainly effusive eruptions of 2004, 2006, and 2008/2009, as well as from the lateral rift-related events of 2001 and 2002/2003.The 2011–2012 paroxysmal activity thus represents an important opportunity to better understand the effects of different magmatic parameters (i.e., P-T-fO2) and magmatic H2O content on the crystallization and fractionation processes. To this aim the petrographic and geochemical features of lava and scoria clasts from 10 paroxysmal events have been investigated. Fractional crystallization modelling indicates that most of the eruptions are related to magmas rising along the vertically-developed feeding system of the volcano, accompanied by one main recharge of a more primitive, deep-seated magma feeding the 4/3/2012 event. Olivine-, clinopyroxene-, and plagioclase-melt equilibria and thermobarometric calculations were performed in order to estimate the crystallization conditions of magmas. These calculations reveal that the erupted products contain different phenocryst populations in equilibrium with a spectrum of primitive to more evolved magma compositions. On the basis of crystal composition, crystal-melt equilibrium conditions and thermobarometric estimations, four main magmatic facies have been recognized: F1, 1600MPa at 1270°C (Ol Fo88); F2, 800MPa to 600MPa at 1178°C to 1151°C (Ol Fo84–78); F3, 450MPa to 250MPa at 1139°C to 1118°C (Ol Fo79–74); F4, <250MPa at <1120°C (Ol Fo75–70). The overall geochemistry and thermobarometric data allow us to characterize the central feeding system as continuous and vertically zoned. During the 2011–2012 activity the studied 30/7/11, 29/8/11, 8/9/11, 18/3/11 and 24/4/12 events were fed by magma residing at F3 and F4 facies. Mafic magma influx from deeper F2 facies occurred 18/2/11 and 20/8/11, with a major recharge event before the 4/3/12 eruption. The primitive magma is testified by rare olivine crystals equilibrated at the F1 facies, located at crust-mantle boundary depth and close to liquidus temperature.

Magmatic processes revealed by anorthoclase textures and trace element modeling: The case of the Lajes Ignimbrite eruption (Terceira Island, Azores)

The Lajes Ignimbrite on Terceira Island (Azores) records the last major pyroclastic density current-forming eruption of Pico Alto Volcano that occurred ca. 21kyrs ago. This comenditic trachyte ignimbrite contains up to 30vol% of crystals, mostly anorthoclase. Geochemical investigation of the products collected throughout two key outcrops reveals that major element compositions are poorly variable, whereas trace elements show significant variability, pointing to the presence of a zoned magma reservoir. Thermometry and oxygen fugacity estimations yielded pre-eruptive temperatures of 850–900°C and ∆NNO from −2.4 to −1.8. Melt–alkali-feldspar hygrometer indicates magmatic H2O contents ranging from 5.8wt% in the upper part of the reservoir to 3.6wt% at the bottom, indicating that the magma reservoir (confined at ~4km depth) was mainly water-undersaturated before the eruption, except for the topmost portion. Two types of anorthoclase crystals were identified. Type 1 crystals show reverse to oscillatory zoning with An contents of 0.4–2.1mol% and Ba of 200–2000ppm. They formed in the middle/upper portion of the reservoir, where fractional crystallization processes dominated. Type 2 crystals, mainly present in the less evolved products, are characterized by patchy-zoned cores with large dissolution pockets surrounded by thick oscillatory-zoned rims and show a wide compositional range (An of 0.5–4.7mol% and Ba of 142–4824ppm). Their zoning patterns, together with whole-rock and glass compositions of the juvenile clasts, are consistent with the involvement of an anorthoclase-bearing cumulate from the bottom of the reservoir that underwent partial melting. Crystal dissolution was likely induced by the presence of a heat source at depth, without any mass transfer to the eruptible magma, as suggested by the lack of petrographic and chemical evidences of mixing between the resident comenditic trachyte and a mafic/intermediate magma. Thermal instability generated convective plumes that were responsible for the admittance of crystals from the cumulate level into the intermediate portions of the magma reservoir and possibly acted as trigger of the explosive eruption.

Can magmatic water contents be estimated from clinopyroxene phenocrysts in some lavas? A case study with implications for the origin of the Azores Islands

Over the last decade there has been renewed interest in determining the water contents of basaltic magmas. A commonly applied method is analysis of H2O from melt inclusions in olivine. However, it is also well known that these can rapidly lose (or gain) H2O by diffusion. An alternative is to measure the H2O contents of clinopyroxene phenocrysts and use a partition coefficient (D) to estimate the original H2O content of the host magma. This approach is not without complications and several recent studies have attempted to assess the effects of diffusive loss of H2O from magmatic clinopyroxenes. In the ideal case, these crystals should be taken from rapidly cooled tephra or lapilli but such materials are not always available. In order to further assess the potential of using 5–10mm clinopyroxenes from lavas we undertook a detailed, multi-analytical investigation of clinopyroxenes from an ankaramite flow on Pico Island in the Azores. We conclude that these can be trusted to preserve (probably minimum) magmatic H2O contents if the H2O concentrations of multiple clinopyroxenes from a single sample form a linear correlation with the AlIV content that demonstrates a coupled substitution with little or no H2O loss. Conversely, if H2O contents decrease from core to rim whereas AlIV contents remain relatively constant then it is likely that those clinopyroxenes lost H2O during differentiation and/or cooling. We suspect that the olivine melt inclusions we analysed from Pico and São Miguel Islands also underwent diffusive loss of H2O. Using these criteria, we present clinopyroxene-derived magmatic H2O estimates for Corvo, Flores Faial, Pico and São Miguel Islands that range from 0.28 to 2.2wt%. When combined with published data these show that H2O contents often extend to higher values on the islands than along the adjacent mid-Atlantic ridge. These localised, elevated H2O contents can explain why the islands are emergent despite being situated away from the ridge and perhaps also the asymmetric nature of the bathymetry of the archipelago. It is possible that this H2O was recycled from material subducted very early on in Earth's history.

A review and update of mantle thermobarometry for primitive arc magmas

![][1] Erupted lavas and tephras remain among the best tools we have to ascertain the mantle processes that give rise to the compositional diversity and spatial distribution of near-primary magmas at volcanic arcs. A compilation of mantle-melt thermobarometry for natural, primitive arc magmas to date reveals published estimates vary between ~1000–1600 °C at ~6–50 kbar. In addition to the variability of mantle melting processes within and between different arcs, this range of conditions is the result of different methodology, such as the nature of reverse fractional crystallization calculations, the choice of thermobarometer, how magmatic H2O was quantified and its calculated effect on pressure and temperature, and choices about mantle lithology and oxygen fugacity. New and internally consistent reverse fractionation calculations and thermobarometry for a representative subset of the primitive arc samples with adequate published petrography, measured mineral and melt compositions, and constraints on pre-eruptive H2O content suggest a smaller range of global mantle-melt equilibration conditions (~1075–1450 °C at ~8–19 kbar) than the literature compilation. The new pressure and temperature estimates and major element modeling are consistent with a model whereby several types of primitive arc magmas, specifically hydrous calc-alkaline basalt, primitive andesite and hydrous high-MgO liquid such as boninite, first form at the location of the water-saturated mantle solidus at pressures of ~20–35 kbar and rise into the hot core of the mantle wedge reacting with the mantle en route. Due to their re-equilibration during ascent, these hydrous magmas ultimately record the conditions in the hot, shallow nose of the mantle wedge at the end of their mantle ascent path rather than the conditions at their point of origin as often interpreted. When the mantle residue for this process is lherzolite, calc-alkaline basalt is generated. When the mantle residue is harzburgite to dunite, either high-Mg primitive andesite or high-MgO liquid is generated, depending on the H2O content. A different type of primitive arc magma, specifically nominally anhydrous arc tholeiite, is generated by near-fractional decompression melting at or near the anhydrous lherzolite solidus in the upwelling back limb of corner flow at ~25–10 kbar and is focused into the same region of the shallow mantle wedge as the hydrous melts. The similarity in the terminus of the mantle ascent paths for both wet and dry primitive arc magmas likely explains their eruption in close spatial and temporal proximity at many arcs. The conditions of last mantle equilibration for primitive arc tholeiites generated by decompression melting also imply that the convecting mantle extends to 10 kbar (~30 km) or less below most arcs. The range of mantle-melt equilibration conditions calculated here agrees well with the range of temperatures predicted for the shallow mantle wedge beneath arcs by geodynamic models, although it suggests some subduction zones may have higher maximum temperatures at shallower depths in the wedge than originally predicted. Primitive hydrous arc magmas also constrain natural variation on the order of 200–250 °C in the maximum temperature in the hot shallow nose of the mantle wedge between arcs. Thus the new primitive magma thermobarometry presented here is useful for understanding melt migration processes and the temperature structure in the uppermost part of the mantle wedge, as well as the origin of different primitive magma types at arcs. [1]: /embed/graphic-1.gif

Remote determinations on fumarole outlet temperatures in an eruptive volcano

AbstractDirect measurement of the fumarole outlet temperature in active volcanoes is impractical. Therefore, we used an aircraft to sample H2 in the volcanic plume ejected from Sakurajima volcano to remotely estimate the highest fumarolic temperatures of the volcano based on hydrogen isotopic fractionation between H2 and magmatic H2O. We successfully estimated that the δD of the fumarolic H2 in September and December 2014 was −135 ± 13‰ and −113 ± 11‰, respectively, and that the corresponding highest outlet temperatures were 1050 ± 120°C and 1199 ± 139°C. Although the temperatures were higher than those determined by using infrared remote sensing, we concluded that they are more reliable estimates of the highest fumarole outlet temperatures. Combined with plume sampling by using aircraft, remote temperature sensing based on the δD of H2 in volcanic plumes can be widely applied to active volcanoes to determine the highest fumarole outlet temperatures.

A Calcium-in-Olivine Geohygrometer and its Application to Subduction Zone Magmatism

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazú, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaOOl/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having ∼1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 μm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis.

Evidence in Tissint for recent subsurface water on Mars

We report unambiguous chemical evidence for subsurface water activity in the martian crust at <600 Ma based on the data from Tissint, a fresh martian meteorite fall with minimal terrestrial weathering. The impact-melt pockets in Tissint contain abundant volatiles (H2O, CO2, F, and Cl), and their concentrations are positively correlated with each other. Higher H2O concentrations also accompany higher deuterium contents. These correlations suggest mixing between two volatile sources. The first source is H2O in the precursor basalt inherited from martian magma. Magmatic H2O in the basalt had low deuterium concentration and was likely stored in the nominally anhydrous minerals. This source contributed little CO2 or halogens to the impact melts. The second source is inferred to be aqueous-alteration products introduced to the basalt by water activity after the basalt erupted. These alteration materials contributed more volatiles to the impact melts than the magmatic source, and had high deuterium abundance, reflecting isotope equilibrium with recent martian atmosphere. The water activities occurred beneath the martian surface after ∼600 Ma (crystallization age), but before ∼1 Ma (ejection age). The chemical and isotopic signatures of the alteration products in Tissint resemble previously known martian samples associated with old water activities on Mars, which can be traced back to ∼4.2 billion years ago (e.g., the mudstone at Gale Crater). This similarity in chemistry and the wide age-span indicate that such water activities were common on Mars throughout its history, which had the potential to form habitable environment. However, the rarity of the volatile-rich zone in Tissint suggests that Martian crustal aqueous processes, where they have occurred are generally limited in their extent of water–rock reaction.

Explosive origin of silicic lava: Textural and [formula omitted]–H2O evidence for pyroclastic degassing during rhyolite effusion

A long-standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava. A widely cited idea is that the explosive-to-effusive transition manifests a two-stage degassing history whereby lava is the product of non-explosive, open-system gas release following initial explosive, closed-system degassing. Direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive, as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent. This explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava. Here we use precise H-isotope and magmatic H2O measurements and textural evidence to demonstrate that effusion results from explosion(s)—lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels (tuffisites) that repetitively and explosively vent from effusing lava. Our measurements show, specifically that D/H ratios and H2O contents of a broad suite of explosive and effusive samples from Chaitén volcano (hydrous bombs, Plinian pyroclasts, tuffisite veins, and lava) define a single and continuous degassing trend that links wet explosive pyroclasts (∼1.6 wt.% H2O, δD=−76.4‰) to dry obsidian lavas (∼0.13 wt.% H2O, δD=−145.7‰). This geochemical pattern is best fit with batched degassing model that comprises small repeated closed-system degassing steps followed by pulses of vapour extraction. This degassing mechanism is made possible by the action of tuffisite veins, which, by tapping already vesicular or brecciated magma, allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed-system domains and large tracts of conduit magma by giving them long-range connectivity. Even though tuffisite veins render magma degassed and capable of effusing, they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut. Thus the effusion of silicic lava, traditionally thought to be relatively benign process, presents a particularly hazardous form of explosive volcanism.

Localized, solid-state dehydration associated with the Varberg charnockite intrusion, SW Sweden

The mineralogy, petrology, and fluid inclusion chemistry of two charnockite patches within a distance of 4–5km of the Varberg magmatic charnockite intrusion, SW Sweden, are investigated and described utilizing SEM, EMPA, and fluid inclusion microthermometry. Garnet–clinopyroxene (890–930°C), garnet–amphibole (600–800°C), and garnet–biotite (670–860°C) Fe–Mg exchange thermometry indicates high temperatures for charnockite Patch I compared to relatively lower garnet–orthopyroxene, garnet–amphibole, and garnet–biotite temperatures of 500 to 600°C for charnockite Patch II. Plagioclase in the charnockitic patches tends to be more anorthitic and less albitic (XAn=0.20, XAb=0.76) than in the surrounding regional granitic gneiss (XAn=0.13, XAb=0.84). Replacement antiperthite is commonly found in unrelated plagioclase grains from either patch compared to the regional granitic gneiss where it is relatively rare. In either patch, K-feldspar is considerably less albitic (XKfs=0.90–0.92, XAb=0.05–0.10) compared to K-feldspar from the regional granitic gneiss. It can also be found as micro-veins along quartz grain rims. Both patches are dominated by clinopyroxene as opposed to orthopyroxene. Garnet, biotite, and amphibole and in both charnockite patches tend to have lower Fe and correspondingly higher Mg values compared with garnet, biotite, and amphibole from the surrounding regional granitic gneiss. Fluorapatite tends to be relatively enriched in Cl and depleted in (Y+REE) compared with fluorapatite from the regional granitic gneiss. Fluid inclusions in charnockite Patches I and II are dominantly carbonic similar to what is seen for the Varberg charnockite. In addition to quartz, relatively high-density carbonic inclusions are also preserved in garnet and in fluorapatite. It is presumed that pure carbonic fluids must have once coexisted with relic magmatic H2O–CO2–NaCl fluids at peak metamorphic conditions. The most likely scenario suggests that charnockite Patches I and II were formed during the later stages of crystallization of the Varberg charnockite magmatic body during which copious amounts of CO2-rich fluids with a brine (CaCl2-dominated) component were expelled into the country rock via pegmatoid segregations both within and in the immediate surroundings of the charnockite body. Patch I appears to represent the extension of a pegmatoid segregation, whereas Patch II appears to represent fluid-induced lower temperature, solid-state dehydration. Transport was facilitated via a system of tectonic fissures and fractures generated in the regional migmatized granitic gneiss during its emplacement. Within the scope of what is known, these two charnockite patches fall into the generally observed parameters for localized dehydration zones in general.

Diabasic intrusion and lavas, segregation veins, and magma differentiation at Kahoolawe volcano, Hawaii

A mafic sill-like intrusion, ~5 × 30 m, exposed along the eastern shoreline of Kahoolawe Island, Hawaii, represents tholeiitic magma emplaced as diabase among caldera-filling lavas. It differentiated from ~7.8 wt.% MgO to yield low-MgO (2.9 wt.%) vesicular segregation veins. We examined the intrusion for whole-rock and mineral compositions for comparison to Kahoolawe caldera-fill lavas (some also diabasic), to the Uwekahuna laccolith (Kilauea), and to gabbros, diabases, and segregations and oozes of other tholeiitic shield volcanoes (e.g., Mauna Loa and Kilauea lava lakes). We also evaluate this extreme differentiation in terms of MELTS modeling, using parameters appropriate for Hawaiian crystallization environments. Kahoolawe intrusion diabase samples have major and trace element abundances and plagioclase, pyroxene, and olivine compositions in agreement with those in gabbros and diabases of other volcanoes. However, the intrusion samples are at the low-MgO end of the large MgO range formed by the collective comparative samples, as many of those have between 8 and 20 wt.% MgO. The intrusion’s segregation vein has SiO2 53.4 wt.%, TiO2 3.2 wt.%, FeO 13.5 wt.%, Zr 350 ppm, and La 16 ppm. It plots in compositional fields formed by other Hawaiian segregations and oozes that have MgO <5 wt.%—fields that show large variances, such as factor of ~2 differences for incompatible element abundances accompanying SiO2 from ~49 to 59 wt.%. Our MELTS modeling assesses the Kahoolawe intrusion as differentiating from ~8 wt.% MgO parent magma beginning along oxygen buffers equivalent to FMQ and FMQ-2, having magmatic H2O of 0.15 and 0.7 wt.% (plus traces of CO2 and S), and under 100 and 500 bars pressure. Within these parameters, MELTS calculates that <3 wt.% MgO occurs at ~1,086 to 1,060 °C after ~48 to 63 % crystallization, whereby the lesser crystallization percentages and lower temperatures equate to higher magmatic H2O, leading to high SiO2, ~56–58 wt.%. To contrast, greater crystallization is calculated for lower H2O, for which it achieves less SiO2, <55 wt.%. While MELTS reliably predicts SiO2 approaching 58 wt.% for differentiation beyond <4 wt.% MgO, and shows that Kahoolawe intrusion’s segregations and those of Kilauea and Mauna Loa are all reasonably accommodated by the modeled parameters and SiO2 differentiation curves, MELTS fails where it predicts that Fe enrichment is more robust under FMQ than FMQ-2 buffers. That failure not withstanding, MELTS differentiation from liquidus temperatures ~1,205–1,185 °C (depending on the various parameters) gradually increases fO2 (up to ~0.4 log units, as normalized to FMQ) until magnetite crystallizes at ~1,090–1,085 °C, which reduces absolute fO2 ~1 to 1.5 log units. The modeled Kahoolawe intrusion, then, exemplifies how tholeiitic magma differentiation can produce extreme SiO2 and incompatible element compositions, and how Hawaiian segregations from shallow intrusions and lava lakes can be generally modeled under compositional and physical parameters appropriate for Hawaiian tholeiitic magmatism.